Vertical oven with a boat for the uniform treatment of wafers

ABSTRACT

Method for the treatment of semiconductor substrates as well as an oven-boat system for this purpose. The oven is embodied as a vertical oven and it is aimed to simultaneously treat a number of substrates arranged one above the other in a boat. To carry out the deposition and such processes at raised temperatures as uniformly as possible, that is, so that each semiconductor substrate substantially undergoes the same treatment, it is proposed on the one hand to vary the ratio of the volume limited by two consecutive armrests and on the other hand the volume limited by screening off the process area and the edge of the substrates from the insertion end of the gas to the discharge end of the gas that flows through the oven.

FIELD OF THE INVENTION

[0001] The present invention relates to an apparatus for the uniformtreatment of wafers wherein a gas or radicals are supplied during thetreatment

BACKGROUND OF THE STATE-OF-THE-ART

[0002] In the state-of-the-art, treatment of groups of semiconductorsubstrates is well-known. Such a process should be distinguished fromsystems wherein the wafers are handled separately, that is, one afterthe other and not simultaneously.

[0003] It is clear that a system wherein a large number of wafers (up toseveral hundreds) are simultaneously treated in a boat, results in aconsiderable time advantage per wafer, especially for long processes.Naturally, the aim is to treat each of the semiconductor substratesidentically, as far as is possible, so that after the treatment allproducts have been treated in the same way and to the same extent,

[0004] One of the problems which can occur is the exhaustion of thetreatment gas, which is introduced at one side of the oven. When a layeris deposited by the treatment gas, for example, his layer will generallybe thicker at the inflow side than at the side where the gas is removed.This effect can be compensated for by choosing a temperature severaldegrees higher on the outflow side than on the inflow side. It will beclear that the application of considerable temperature differences inthe oven is undesirable, since a difference in treatment does thenoccur. A particular case occurs with the depositing of silicon dioxidelayers from Tetra Ethyl Ortho Silicate (TEOS) vapor. In this case it hasbeen found that, for a relatively high process pressure, hightemperature and low TEOS gas flow, the layer thickness on the gasoutflow side is smaller than on the gas inflow side, as is to beexpected due to the exhaustion of the treatment gas. Nonetheless, forlow process pressure, low temperature and high TEOS gas flow, the layerthickness on the outflow side can be thicker than on the inflow side.Likewise, it has been observed that the layer thickness at the edge ofthe wafer, compared with the layer thickness at the center of the wafer,increases in the direction of the outflow side: depending on the chosentemperature, pressure and TEOS gas flow, the substrate near to the gasinflow side can show a pronounced convex profile in the layer thickness,that is, thinner at the edge than in the center, while under the samecircumstances, a substrate near the gas outflow side can show apronounced concave profile.

[0005] The result is that no uniform treatment takes place and, forcertain applications, parts of the boat are not filled with wafers,because such an uneven deposition and reaction respectively take placeeither in the top part or in the bottom part of the boat, compared withthe other parts of the boat, that semiconductor substrates are producedwhich are no longer usable.

[0006] The decomposition of TEOS is described in J. Electrochem. Soc.,Vol. 140, No. 10, October 1993, page 2952, by T. Sorita, S. Shiga, K.Ikuta, Y. Egashira and H. Komiyama. These authors suggest that thedecomposition of the TEOS molecule takes place via the forming ofintermediate radicals in the gas phase. These intermediate radicals inturn attach themselves to the wafer and form the silicon dioxide layer.The deposition rate of the silicon dioxide layer is directly related tothe concentration of the intermediate radicals in the gas phase. Toachieve a uniform layer thickness over the whole series of semiconductorsubstrates, the concentration of these intermediate radicals must beconstant across the whole reactor, This concentration is determined bythe balance between the rate with which the intermediate radicals areformed and the rate with which they are consumed by deposition on thesubstrates. The rate of the forming is determined by the processconditions, such as pressure, TEOS concentration, temperature and theconcentration of already present intermediate radicals. Theabove-mentioned process results can now be explained as follows. At lowpressure, low temperature and high TEOS gas flow, the residence time atthe inflow side was too short to build up a sufficient concentration ofintermediate radicals. Consequently, under these circumstances, thelayer thickness of the inflow side is relatively thin and the layerthickness at the edge of the substrates is relatively thin. During theinward diffusion, a higher concentration of intermediate radicalsalready forms whereby the layer thickness near the center of thesubstrate increases. Near the gas outflow side, a high concentration ofintermediate radicals has already formed while the exhaustion of theTEOS itself is relatively small. This results in higher deposition speedand a relatively thicker edge, At high pressure, high temperature andlow TEOS gas flow, the forming of the intermediate radicals is no longera limiting factor as the residence time of the gas in the reactor isincreased. In this case, the detected layer thickness will show theusual exhaustion characteristics of TEOS: thinner towards the center ofthe substrates and thinner towards the gas outflow side.

[0007] It is noted that the rate of forming of the intermediate radicalsin the gas phase is a volume measure. With a higher gas volume, anproportionally larger number of intermediate radicals will form, inabsolute terms. The ‘consumption’ of intermediate radicals is determinedby the conversion rate in the silicon dioxide layer, and this depends onthe temperature and the concentration of the intermediate radicalspresent. For the decrease in absolute terms, it is the amount ofavailable surface on which the silicon dioxide can form that isimportant. To summarize, it can be concluded that for a uniform processresult, it is important that a balance exists throughout the reactorbetween the forming of the intermediate radicals, which increaseproportionally with the locally available gas volume, and theconsumption of the intermediate radicals, which increases proportionallywith the available surface. This surface is made up of the surface ofthe process tube, the boat and the substrates. A balance between volumeand surface is thus of fundamental importance. Near the center of thesubstrate, the ratio of surface to volume is entirely determined by thedistance between the substrates. At the edge of the substrates, besidesthe distance between the substrates, the amount of space between thesubstrate edge and the process tube is also of importance.

[0008] In the abstract of the Japanese patent application 56155529, aboat with a variable pitch is described. With this, according to thedescription a more even thickness is achieved. In this system thesubstrates are loaded by hand.

[0009] In the U.S. Pat. No. 5,217,560, gas or radicals are introducedover the height of the boat from the outside by various openings, toachieve an equal concentration in this way. In this manner, anoptimization of the uniformity of the layer thickness can be achievedfrom the inflow side to the outflow side, but not from the edge to thecenter of the wafer. Furthermore the complexity of the system increasesby the extra gas supply openings and, when used, the amount of gas to besupplied to each of the individual gas openings must be determined andcontrolled. Such a process with so many parameters is more liable tofaults and is undesirable in production circumstances.

SUMMARY OF THE INVENTION

[0010] An object of the invention is to avoid these disadvantages and toprovide an oven-boat system wherein the variation of layer thickness,both for the height and cross-section of the oven-boat system, islimited as much as possible and wherein the wafers can be loaded by arobot without the necessity to program the position of each individualwafer. Likewise the invention aims to provide a boat which can be usedwith such an oven-boat system. Furthermore, the invention aims toprovide an oven with which the above mentioned can be met.

[0011] According to an aspect of the invention, an oven-boat system isprovided for the treatment of a number of semiconductor substrates,arranged in a spaced array in an oven, wherein the oven is provided atone end with an insertion opening with a lining tube for delimiting atreatment area, and externally provided with heating elements and aninsulating cover wherein means are provided for the insertion of atreatment gas from said end, and means for the removal of gas from saidend of the oven, and whereby the boat comprises a number of mutuallyspaced supports to carry the semiconductor substrates fitted to theframe of the boat such that said disk shaped semiconductor substratessubstantially extend in sequence, wherein the distance (pitch) betweenthe consecutive supports changes from the insertion end of the treatmentgas to the discharge end in such a way that the difference in distancebetween adjacent semiconductor substrates is substantially constant.This makes the programming of the robot relatively simple.

[0012] According to a further aspect of the invention, an oven-boatsystem is provided for the treatment of a number of semiconductorsubstrates arranged in a spaced array in an oven, wherein the oven isprovided at one end with an insertion opening which is provided with alining tube for the confining therein of a treatment area, andexternally provided with heating elements and an insulating coveringwherein means are present for the insertion of a treatment gas from thatend and means for the removal of gas from that end of the oven, andwherein the boat comprises a number of mutually spaced supports, tocarry the semiconductor substrates fitted to the frame of the boat suchthat those disk shaped semiconductor substrates substantially extend insequence, wherein the cross section surface area of the lining tubechanges from the insertion end of the treatment gas to the gas removalend.

[0013] According to still a further aspect of the invention, a boat isprovided for use in an oven-boat system as described above, comprising anumber of mutually spaced supports to carry the semiconductor substratesfitted to the frame of the boat, such that those disk shapedsemiconductor substrates substantially extend horizontally and arearranged one above the other in the position of use, wherein thedistance (pitch) between the consecutive supports increases from theinsertion end of the treatment gas to the gas removal end.

[0014] The invention will be elucidated further with reference to anexample embodiment shown in the drawings. Wherein:

[0015]FIG. 1. schematically shows a first embodiment of the oven-boatsystem according to the invention;

[0016]FIG. 2. shows a second embodiment of the oven-boat systemaccording to the invention;

[0017]FIG. 3. shows the position of the measuring points on the wafers,

[0018]FIG. 4. shows the results of experiments carried out with theoven-boat system according to the state of the art, wherein the wafershave a constant distance between them and the lining tube has a constantdiameter and

[0019]FIG. 5 shows the results of experiments carried out with anoven-boat system according to FIG. 1.

[0020] In FIG. 1 a first embodiment of the oven according to theinvention is in generally referred to by 1. This is shown here in a veryschematic form, and only shows those parts which are essential for theunderstanding of the invention. Oven 1 consists of the lining tube 2which is closed at the top side 3 with the exception of the outlet 6from which the gas outlet 7 branches off. On the underside, the verticaloven is provided with a gas inlet 4. The underside of the oven isprovided with an opening which can be closed with a lid 5. On theoutside is the lining tube 2, which can be made from a quartz material,silicon carbide or such like, provided with a heating element 8 as wellas insulation 9.

[0021] An insulation plug 10 is fitted on lid 5 on which the boat 19 isplaced. This boat 19 consists of a frame 11 provided with a number ofsupports 12-17 to carry the semiconductor substrates 20.

[0022] It is clear from FIG. 1 that the distance between supports 16 and17 is smaller than the distance between supports 14 and 15 which in turnis smaller than the distance between supports 12 and 13. That is, fromtop to bottom the distance between the supports, and thus the distancebetween the semiconductor substrates 20 (wafers) which rest on these,decreases, By the moving of gas from gas inlet 4 to gas outlet 7, whichgas forms intermediate radicals in the gas phase, the exhaustion of thesupplied gas will be compensated for by the larger distance between thesemiconductor substrates whereby forming and consumption of theintermediate radicals remains in balance, the concentration ofintermediate radicals across the reactor remains substantially constantand the variation of treatment across the oven is limited as much aspossible.

[0023] The difference in distance occurs with a substantially equalvalue. Consequently it is possible to program the robot in a relativelyeasy manner. During the moving of the wafers in and out of the boatsystem, when moving to the next position, the robot must take intoaccount the changed distance which increases or decreases by a constantvalue.

[0024] A similar effect can be reached using the construction accordingto FIG. 2 and/or in combination with the system described above. Theoven shown there is indicated by 21 and is provided with a lining tube22. This is tapered, as is clear from the drawing. The closed top sideis indicated by 23 and from there a gas outlet 26 extends which has abranch 27. Gas is supplied via pipe 24 and a lid 25 is present to closethe opening on the underside of the oven, with the help of which lidboat 29 is moved in and out of the oven, whereby this boat 29 ispositioned on the insulation plug 30 on which the frame 31 of the boatrests. In contrast to the embodiment described above, the distancebetween the supports 32 fitted on the frame 31 to carry thesemiconductor substrate 20 is constant.

[0025] During the treatment of the wafers with TEOS, with the gas supplyat the underside of the reactor, the layer thickness at the edge of thewafer at this underside of the reactor remains relatively thin incomparison to the layer thickness at the center of the wafer, At the topside of the reactor, however, the layer thickness at the edge of thewafer is relatively thick. By increasing the volume of gas outside thewafer at the bottom of the reactor and decreasing the volume of gas atthe top of the reactor, the variation in layer thickness across theindividual wafers is limited.

[0026] Likewise it will be understood that for other processes,precisely the opposite measures would have to be taken, that is, whengas passes through, as sketched in the figures, the distance between thewafers in the vertical direction must become smaller or the lining orthe oven must taper in the upward direction.

[0027] The invention will be further elucidated below with the help ofthe example embodiments.

[0028] First, the TEOS process was carried out, wherein silicon oxidewas deposited on semiconductor substrates embodied in oven-boat systemaccording to the state-of-the-art with the semiconductor substratesspaced out with a constant distance between the disks of 4.78 mm. Thediameter of the substrates was 200 mm and there were 167 of them. Thetemperature was 675° C., the pressure 750 m Torr, while the TEOS gasflow was 200 sccm. When the process had finished, layer thicknessmeasurements were carried out using a PLASMOS ellipsometer. The layerthickness profile across the wafer was determined by measuring the layerthickness at 5 points, as shown in the FIG. 3. Points 1 and 5 are at adistance of 6 mm from the edge of the wafer. The relative layerthickness is plotted in FIG. 4 for three wafers which are at the gasinflow side of the boat, slot 10, halfway up the boat, slot 85, and atthe gas outflow side, slot 160. The wafer in slot 10 shows a convexlayer thickness profile with a variation in excess of ±/−4% while thewafer in slot 160 shows a concave layer thickness profile with avariation in excess of ±/−3%.

[0029] Next, the TEOS-process was carried out with an oven-boat systemaccording to FIG. 1., wherein the other conditions were chosen as statedabove. The relative layer thickness is shown in FIG. 5. The sharpdecrease in variation of the layer thickness when compared to FIG. 4 isclear.

[0030] In the treatment of wafers with TEOS as described above, the gasis supplied at the underside of the reaction. During the supply of thisgas, a reaction occurs whereby the intermediate product is formed. Atthe inlet for the gas (underside) less intermediate product is presentthan at the top side (outlet). Consequently, without further measurebeing taken, an unequal deposition takes place. Furthermore, for thewafers at the under side of the oven, the concentration of theintermediate product will increase from the edge to the center. For thewafers at the top of the oven, the concentration of the intermediateproduct from the wall to the center will decrease. By taking thepreviously described measures, this concentration gradient can belargely counteracted.

[0031] Likewise, it will be understood that for other processes,precisely the opposite measures would have to be taken, that is, whengas passes through, as sketched in the figures, the distance between thewafers in the vertical direction would have to be smaller or the liningof the oven would have to taper in the upward direction.

[0032] The amount of available treatment gas in the center of the disk(and thus the amount of deposition in the center) is mainly determinedby the volume between the disks; a larger volume, and thus a largerdistance between the disks will mean more available treatment gas.

[0033] The amount of available treatment gas at the edge of the disk ismainly determined by the volume found between the disks and theshielding of the process area.

[0034] By independently varying the volume of treatment gas at the edgeof the disks and the of treatment gas in the center of the disks, thedifference in the amount of deposition at the edge and the volume ofdeposition in the center can be decreased.

[0035] The described change of the amount of volume can for example takeplace in two ways, whether or not at the same time. First it possible toenlarge the distance between the semiconductor substrates, that isbetween the supports. In this way the concentration gradient across thesurface of the semiconductor substrate also decreases, Anotherpossibility is the conical or stepped embodiment of the oven tube, sothat the distance to the edge of the semiconductor substrate concernedbecomes larger or smaller, whereby the available volume from which gasmolecules can be deposited, or take part in the reaction in another way,changes.

[0036] It is however also possible to fit a dummy surface in some wayaround the semiconductor substrate so that any exhaustion of the gasnear the edge of semiconductor substrate takes place by reacting withthis dummy surface and a more uniform concentration is present above thesubstrate. Such a dummy surface can be present on both the boat and theliner.

[0037] The oven-boat system described above can be used for theembodiment of all reactions known in the state-of-the-art, wherein theconcentration of the gas or intermediate radicals near the semiconductorsubstrate is important. Here, the so-called TEOS process is particularlyconsidered, but not exclusively. This is a deposition process wherebytetraethylorthosilicate is used as the treatment gas to deposit silicateon a semiconductor substrate.

[0038] It will be understood that the earlier mentioned measures can becombined to achieve optimal uniformity. Furthermore it is possible tovary the conical form and the pitch

[0039] and the change therein, as required by the process. Likewise itis possible to fit an auxiliary surface near the edge of thesemiconductor substrate concerned on which material is deposited,whereby the concentration gradient in the horizontal surface of thesemiconductor disk is limited as much as possible.

[0040] These and further variants are within the reach of those skilledin the state-of-the-art after reading the description above and fallwithin the scope of the attached claims.

1. An oven-boat system for the treatment of a number of semiconductorsubstrates arranged in a spaced array in an oven, wherein the oven isprovided at one end with an insertion opening which is provided with alining tube for delimiting a treatment area and externally provided withheating elements and an insulating cover wherein means are provided forthe insertion of a treatment gas from said end and means for the removalof gas from said end of the oven and wherein the boat comprises a numberof mutually spaced supports, to carry the semiconductor substratesfitted to the frame of the boat such that said disk shaped semiconductorsubstrates substantially extend in sequence, wherein the distance(pitch) between the consecutive supports changes from the insertion endof the treatment gas to the discharge end, in such a way that thedifference in distance between adjacent semiconductor substrates issubstantially constant.
 2. An oven-boat system according to claim 1 ,wherein the cross section surface area of the lining tube changes fromthe insertion end of the treatment gas to the gas discharge end.
 3. Anoven-boat system according to claim 2 , wherein the lining tube isconically shaped.
 4. An oven-boat system according to claim 1 , whereinthe treatment gas comprises a deposition gas.
 5. An oven-boat systemaccording to claim 4 , wherein the treatment gas comprisestetraethylorthosilicate.
 6. An oven-boat system according to claim 1 ,wherein the cross section surface area of the lining tube decreases fromthe insertion end of the treatment gas to the gas discharge end.
 7. boatfor use in an oven-boat system according to claim 1 , comprising anumber of mutually spaced supports to carry the semiconductor substratesfitted to the fame of the boat, such that those disk shapedsemiconductor substrates substantially extend horizontally and arearranged one above the other in the position of use, wherein thedistance (pitch) between the consecutive supports increases from theinsertion end of the treatment gas to the gas removal end such that thedifference in the distance between adjacent semiconductor substrates issubstantially constant.
 8. An oven-boat system for the treatment of anumber of semiconductor substrates arranged in a spaced array in anoven, wherein the oven is provided at one end with an insertion aperturewhich is provided with a lining tube for delimiting a treatment area andexternally provided with heating elements and an insulating coveringwherein means are present for the insertion of a treatment gas from thatend and means for the removal of gas from that end of the oven andwherein the boat comprises a number of mutually spaced supports, tocarry the semiconductor substrates fitted to the frame of the boat suchthat those disk shaped semiconductor substrates substantially extend insequence, wherein the cross section surface area of the lining tubechanges from the insertion end of the treatment gas to the gas dischargeend.
 9. An oven-boat system according to claim 8 , wherein the liningtube is embodied as a cone.
 10. An oven-boat system according to claim 8, wherein the treatment gas comprises a deposition gas.
 11. An oven-boatsystem according to claim 10 , wherein the treatment gas comprisestetraethylorthosilicate
 12. An oven-boat system according to claim 8 ,wherein the cross section surface area of the lining tube decreases fromthe insertion end of the treatment gas to the gas removal end.
 13. Anoven for use in a system according to claim 8 for the treatment of asemiconductor substrates wherein the oven comprises a vertical ovenprovided on the underside with an insertion/removal aperture with alining tube for delimiting a treatment area and externally provided withheating elements and an insulating covering wherein means are presentfor the insertion of a treatment gas from the top or underside of theoven and wherein means are present for the discharge of gas from the topor underside of the oven, wherein the cross section surface area of thelining tube decreases from the insertion end of the treatment gas to thegas discharge end.